The present invention relates to a single-wire dryer group of a paper making machine and particularly to air deflection means within a pocket of the dryer group for generating a vacuum there.
A single wire dryer group has a plurality of dryer cylinders. The web to be dried moves over the cylinders in sequence. A respective guide roll is disposed between each two cylinders in the path of the web. A porous support belt, e.g. a wire screen or wire, carries the web to be dried from one dryer cylinder to a succeeding guide roll and then to the next dryer cylinder in the path of the web through the dryer group. The web is carried on the side of the support belt that places the web in direct contact with the dryer cylinders and that also places the web on the outside of the support belt as it passes over the guide rolls. Features of such a single wire dryer group are disclosed in U.S. Pat. No. 4,359,828, particularly FIG. 7, for example. Such a dryer is preferably part of a paper making machine.
In a single-wire dryer group, the web to be dried, which is a web of paper, is guided continuously by the porous support belt and is pressed directly against the dryer cylinders. The web and the support belt travel, for instance, from a first dryer cylinder jointly over the shortest possible path to the following guide roll and from the latter back to a second dryer cylinder and then over a second guide roll to a third dryer cylinder, etc. The volume bounded by two neighboring dryer cylinders and the guide roll between them is called the pocket. Typically, the guide roll axis is off an imaginary line between the axes of the two cylinders and the pocket is to the side of that line toward the guide roll axis.
The guide roll can be arranged in a symmetrical arrangement at an equal distance from each of the two adjacent dryer cylinders. However, an asymmetric arrangement is known in which the smallest possible distance is provided between the first dryer cylinder and the guide roll while a substantially greater distance is provided between the guide roll and the second dryer cylinder.
Upon the web leaving the first dryer cylinder, it is essential that the web not adhere to the cylinder, but that the web instead be drawn against the support belt by a vacuum produced in the pocket. Also, the web must be drawn against the support belt in the wrapping zone around the guide roll, in opposition to the centrifugal force acting on the web. This can be achieved, for instance, by producing the guide roll with circumferential grooves into which the vacuum produced in the pocket is propagated. Alternatively, the guide roll is formed as a suction roll, with or without an inner stationary suction box, and the shell of the roll is perforated. It is also important that the web be dependably held against the support belt along the path of the web from the guide roll to the second dryer cylinder by the vacuum prevailing in the pocket.
In FIG. 7 of U.S. Pat. No. 4,359,828, production of a vacuum in the pocket is attempted by the following measures. A first sealing ledge, which is fastened to a sealing ledge support, is arranged in the pocket in the vicinity of the outer surface of the first dryer cylinder. Similarly, a second sealing ledge, which is also fastened to a sealing ledge support, is arranged in the vicinity of the outer surface of the second dryer cylinder. The two sealing ledge supports are curved in such a manner that they deflect the flow of air that is induced by the traveling support belt on the first dryer cylinder into the direction in which the support belt travels over the second dryer cylinder. Together with an additional flow guide wall, the two sealing ledge supports form a flow channel which has an ejector like opening through which air is drawn upward out of the pocket. In this way, a certain vacuum can prevail in the pocket. The strength of this vacuum is, however, insufficient in actual practice.